The inner ear's vestibular system provides cues about self-motion that help stabilize vision during movement. These cues also enable us to orient ourselves with respect to our surroundings, which helps us to stand and walk. Each inner ear can sense, in 3-D, angular motion and the sum of forces due to linear acceleration and gravity (V. Wilson, B. Peterson, et al., “Analysis of vestibulocollic reflexes by sinusoidal polarization of vestibular afferent fibers,” Journal of Neurophysiology, Vol. 42, No. 2, 1979, p. 331-46). The central nervous system can process these motion cues to estimate self motion in 6 degrees of freedom: three angular and three linear. When a malfunction occurs in the inner ear, the neural pathways that connect the inner ear to the central nervous system, or the part of the central nervous system that processes self-motion information, due to injury, disease, or to prolonged exposure to altered gravity, motion cues are lost or distorted. This lack of accurate sensory information can result in dizziness, blurred vision, inability to orient correctly (including the ability to align with the vertical), and reduced ability to stand or walk, especially under difficult conditions.
Vestibular or balance prostheses have been developed in the hope of improving postural stability in the balance impaired. Basic uses for balance prostheses include: (1) a vestibular “pacemaker” to reduce dizziness and imbalance due to abnormal fluctuations in the peripheral vestibular system, (2) permanent replacement of vestibular function, (3) temporary replacement of motion cues that commonly occur following ablative surgery of the inner ear, and (4) vestibular/balance rehabilitation.
Balance prostheses may be implantable or non-implantable. An implantable prosthesis delivers self-motion cues to the central nervous system via implanted stimulators. Non-implantable prostheses are a less invasive means of providing some self-motion cues. Such prostheses operate by, for example, stimulating the vestibular nerve via surface electrodes or by displaying self-motion cues using “sensory substitution” (e.g., acoustic inputs or electric currents applied to the tongue). (See P. Bach-y-Rita, “Late post-acute neurologic rehabilitation: neuroscience, engineering and clinical programs,” Arch Phys. Med. Rehab, Vol. 84, No. 8, 2003, p. 1100-8, and P. Bach-y-Rita, K. A. Kaczmarek, et al., “Form perception with a 49-point electrotactile stimulus array on the tongue: a technical note,” J Rehabil Res Dev. Vol. 35, No. 4, 1998, p. 427-30.) Stimulation using auditory cues is described in U.S. Pat. No. 5,919,149 (“the '149 patent”), the full disclosure of which is incorporated by reference herein.
U.S. Pat. No. 6,546,291, the full disclosure of which is incorporated herein by reference, describes vestibular prostheses that include tactile vibrators (tactors) mounted on the subject's torso. Several generations of this type of prosthesis have been tested and have reduced sway in vestibulopathic subjects. Initial, single axis tests were performed, with the subject receiving only information about forward (or sideward) motion. A particularly noteworthy result was the ability of vestibulopathic subjects deprived of visual and proprioceptive inputs to stand without falling. (M. S. Weinberg and C. Wall, “MEMS Inertial Sensor Assembly for Vestibular Prosthesis,” The Institute of Navigation 59th Annual Meeting, Albuquerque, N. Mex., Jun. 23-25, 2002; M. Weinberg and C. Wall, “Sensor Assembly for Postural Control Balance Prosthesis,” Transducers '03, Boston, Mass., June 9-12; J. Vivas, “A Precursor to a Balance Prosthesis via Vibrotactile Display,” Mass. Institute of Technology Masters Thesis, May, 2001; D. Merfeld, S. Ranch, et al., Vestibular Prosthesis Based on Micromechanical Sensors, U.S. Pat. No. 6,546,291, Apr. 8, 2003; and E. Kentala, J. Vivas, and C. Wall III, “Reduced Postural Sway by use of a Vibrotactile Balance Prosthesis,” Ann Otol Rhinol Laryngol, Vol. 5, No. 112, 2003.)